This invention relates to optical fiber networks and, more particularly, to remotely sensing the occurrence and determining the location of a cyclic disturbance that affects a localized portion of an optical fiber included in such a network.
As the use of optical fibers in communications systems continues to grow, it is becoming increasingly important that reliable low-cost techniques be available for monitoring the condition of the fibers. Thus, for example, during installation of a fiber, testing is carried out to ensure that splices and connectors have low loss, that excessive losses have not been introduced due to bending or stressing of the fiber, and that the total end-to-end loss of the fiber is within prescribed specifications. Further, during actual operation of an installed fiber, regular checking is typically carried out to determine whether or not some degradation in the condition of the fiber has occurred. And, if the fiber is found to be impaired (accidentally cut, for example) it is important to be able to quickly and inexpensively ascertain the location of the impairment.
It is well known that information concerning the condition of an optical fiber can be obtained by repetitively launching optical pulses into one end of the fiber and then analyzing light that is backscattered from the fiber. This conventional single-ended technique, known as optical time domain reflectometry (OTDR), is well established as a useful tool for monitoring optical fiber networks.
Standard OTDR techniques make use of the information contained in the overall intensity of the backscattered light from an optical fiber to locate, for example, bad fiber-to-fiber joints and anomalously lossy sections of the fiber. It has also been recognized that there are many external influences (magnetic field, electric field, stress, strain, temperature, etc.) which act to change the polarization state of the light propagating in the fiber. Polarization state can be determined with an OTDR system modified to be polarization sensitive. (See, for example, "Polarization-Optical Time Domain Reflectometry: A Technique for the Measurement of Field Distributions", by A. J. Rogers, Applied Optics, Vol. 20, No. 6, pages 1060-1074, Mar. 15, 1981.) In general, the detected backscattered signal is very faint, and monitoring techniques based on OTDR or on polarization-sensitive OTDR (POTDR) require extensive time-averaging to produce a result.
Heretofore, monitoring techniques based on OTDR or on POTDR have not been recognized as being suitable for sensing the occurrence of a cyclically recurring disturbance near a localized portion of an optical fiber. In practice, such monitoring is important to detect the potential impairment of a buried optical cable by, for example, digging equipment. If the presence of a periodic vibration indicative of digging equipment in the vicinity of the cable can be detected in time to initiate some corrective action, a potentially serious disruption of communications service, due, say, to cracking or breaking of the cable, can be avoided.
In the aforecited Rogers article, the applicability of POTDR techniques to monitoring vibrational modes in the vicinity of an optical fiber is recognized. But the techniques described therein rely solely on the inducement by the vibrations of spatially distributed standing waves along the longitudinal axis of the fiber. Thus, neither the POTDR techniques of Rogers nor known OTDR/POTDR techniques are amenable to sensing the potential impairment of an optical fiber by detecting the occurrence and determining the location of particular cyclic disturbances in the vicinity of a localized portion of the cable.
Accordingly, efforts have continued by workers skilled in the art directed at trying to improve the capability of sensing techniques for optical networks. In particular, these efforts have been directed at trying to devise a reliable way of detecting the occurrence and determining the location of periodic disturbances near a localized portion of an optical cable. It was recognized that these efforts, if successful, could detect a potential impairment to the cable and thereby provide a basis for corrective action before communications service is degraded or interrupted. Such a capability would, of course, significantly improve the reliability and lower the cost of maintaining optical fiber networks.